1. Field of the Invention
The present invention relates to left-right independent steering devices for steering left and right wheels independently by controlling individual lengths of tie rods by extraction/contraction.
2. Description of the Related Art
With the aim of improving steering stability, usability, and stability of vehicles such as automobiles, there have been developed devices that can independently adjust turning angles or toe angles of left and right wheels. Recently, various types of systems have been developed that no longer have a mechanical linkage between the turning wheels and the steering wheel. One typical example is a steer-by-wire system in which steering control is performed by electrically detecting a driver's steering operation.
However, steer-by-wire systems have various problems to be solved due to being insufficient in reliability, requiring installation of a high-voltage (e.g. DC 42 V) power source, and requiring implementation of appropriate control of steering reaction force. For these reasons, a new vehicle design is required to apply a steer-by-wire system. Therefore, left-right independent steering technologies are still predominantly realized based on the conventional devices or technologies that apply the mechanical linkage between the turning wheels and the steering wheel. An example of such a left-right independent steering device is disclosed in Japanese Unexamined Patent Application Publication No. 2006-56374.
In the steering device disclosed in Japanese Unexamined Patent Application Publication No. 2006-56374, auxiliary rack bar mechanisms that advance or recede in response to rotation of a motor are provided at opposite ends of a rack bar that extends leftward and rightward from a gear box located at a lower end of a steering shaft. These auxiliary rack bar mechanisms transmit their advancing or receding motion to respective tie rods so that the left and right wheels can be steered individually with knuckle arms. When a difference in operation forces acting on the left and right turning wheels during forward movement of the vehicle is greater than or equal to a predetermined value, the steering amount of at least one of the left and right turning wheels is controlled by driving the corresponding auxiliary rack bar mechanism. Accordingly, this can reduce the effect of the operation forces acting on the turning wheels from the road surface.
However, the steering device disclosed in Japanese Unexamined Patent Application Publication No. 2006-56374 has difficulties in view of mountability. Specifically, since the rack bar extending from the opposite sides of the gear box is provided with rod-length adjusting mechanisms and actuators (motors), the foot space for the driver at the vehicle interior side may have to be reduced in order to avoid interference with peripheral components such as suspension arms and stabilizers. This implies that significant modifications to existing vehicles or development of dedicated vehicles will need to be made, thus unfavorably eliminating the capability to use existing devices and technologies.
Furthermore, in order to perform independent steering control of left and right wheels by driving the respective actuators, it is necessary to obtain accurate and highly reliable information about actual turning angles of the wheels. Japanese Unexamined Patent Application Publication No. 2006-56374 does not particularly discuss this point and therefore leaves room for improvement.